The present invention relates to filters. More particularly, the invention relates to a method and apparatus using micro electro mechanical system (MEMS) technology for tuning a filter.
Several types of filters are commonly used in electronic applications. These filters include, for example, high-pass filters, low-pass filters, band-pass filters, and band-stop filters. Each filter type provides a specific filtering function to meet a required performance characteristic.
The above-mentioned filters are well known in the art and will not be discussed in detail. Briefly, a high-pass filter has a passband from some frequency xcfx89p up upward, and a stopband from 0 to xcfx895 (where xcfx89s less than xcfx89p). Conversely, a low-pass filter has a passband from 0 to xcfx89p, and a stopband from xcfx89s upward (where xcfx89p less than xcfx89s).
Band-pass and band-stop filters are similar to high-pass and low-pass filters, but include additional cutoff frequencies to accommodate the added filtering criteria. For example, a band-pass filter has a passband from xcfx89p1 to xcfx89p2, and a stopband from 0 to xcfx89s1 and xcfx89s2 upward (where xcfx89s1 less than xcfx89p1 less than xcfx89p2 less than xcfx89s2). Conversely, a band-stop filter has a passband from 0 to xcfx89p1 and from xcfx89p2 upward, and a stopband from xcfx89s1 to xcfx89s2 (where xcfx89p1 less than xcfx89s1 less than xcfx89s2 less than xcfx89p2).
The need for a high-quality factor (Q), low insertion loss tunable filter pervades a wide range of microwave and RF applications, in both military, e.g., radar, communications and electronic intelligence (ELINT), and commercial fields such as in various communications applications, including cellular. For example, placing a sharply defined band-pass filter directly at the receiver antenna input will often eliminate various adverse effects resulting from strong interfering signals at frequencies near the desired signal frequency in such applications. Because of the location of the filter at the receiver antenna input, however, the insertion loss must be very low to not degrade the noise figure. In most filter technologies, achieving a low insertion loss requires a corresponding compromise in filter steepness or selectivity.
In many applications, particularly where frequency hopping is used, a receiver filter must be tunable to either select a desired frequency or to trap an interfering signal frequency. Thus, the insertion of a linear tunable filter between the receiver antenna and the first nonlinear element (typically a low-noise amplifier or mixer) in the receiver offers, providing that the insertion loss is very low, substantial advantages in a wide range of RF and microwave systems. For example, in radar systems, high amplitude interfering signals, either from xe2x80x9cfriendlyxe2x80x9d nearby sources, or from jammers, can desensitize receivers or intermodulate with high-amplitude clutter signal levels to give false target indications. In high-density signal environments, RADAR warning systems frequently become completely unusable.
Micro Electro-Mechanical Systems (MEMS) technology is currently implemented for the fabrication of narrow band-pass filters (high-Q filters) for various communication circuits (see U.S. Pat. No. 6,275,122 issued to Speidell et al.). These filters use the natural vibrational frequency of micro-resonators to transmit signals at very precise frequencies while attenuating signals and noise at other frequencies. A conventional MEMS band-pass filter device includes a semi-conductive resonator structure suspended over a conductive input structure, which is extended to a contact. By applying an alternating electrical signal on the input of the device, an image charge is formed on the resonator, attracting it and deflecting it downwards. If the alternating signal frequency is similar to the natural mechanical vibrational frequency of the resonator, the resonator may vibrate, enhancing the image charge and increasing the transmitted AC signal. The meshing of the electrical and mechanical vibrations selectively isolates and transmits desired frequencies for further signal amplification and manipulation.
Tuning the resonator frequency in the above described MEMS filter can be implemented by applying a DC bias voltage relative to the input contact, which will apply an internal stress to the resonator. Alternatively, a DC bias voltage can be applied relative to the output contact which will cause a current to flow through the resonator, thus increasing its temperature. Both types of bias change the modulus of elasticity of the resonator, resulting in a change of its fundamental natural vibrational frequency and therefore changing the filter characteristics.
A drawback to this approach of tuning the resonator frequency is that there are numerous variables that must be taken into consideration to determine the change in resonator frequency. These variables include, for example, the actual current injected into the device, the actual temperature rise of the device due to the injected current, elasticity variations of the resonator, and the ambient temperature. A slight error, for example, in the calculation of the temperature rise or in the effect of the ambient temperature may result in an error in the tuning frequency and thus less than optimal performance of the filter.
Tunable filters also have been implemented using a micro electro mechanical (MEMS) variable capacitor, wherein the capacitance is altered by changing the distance between the capacitor plates. In the simple vertical motion, parallel plate form of this device, a thin layer of dielectric separating normal metal plates (or a normal metal plate from very heavily doped silicon) is etched out in processing to leave a very narrow gap between the plates. The thin top plate is suspended on four highly compliant thin beams which terminate on posts (regions under which the spacer dielectric has not been removed). When a DC tuning voltage is applied between the plates, the small electrostatic attractive force, due to the high compliance of the support beams, causes substantial deflection of the movable plate toward the fixed plate or substrate, thus increasing the capacitance.
While the conventional MEMS variable capacitor structure is capable of improved Q values and avoids intermodulation problems of xe2x80x9ctunable materialsxe2x80x9d, it has some potential problems. Because only the relatively weak electrostatic attraction between plates is used to drive the plate motion to vary the capacitance, the plate support xe2x80x9cspiderxe2x80x9d structure must be extremely compliant to allow adequate motion with supportable values of bias voltage. A highly compliant suspension of even a small plate mass may render the device subject to microphonics problems (showing up as fluctuations in capacitance induced by mechanical vibrations or environmental noise). Having the electric field which drives the plates directly in the signal dielectric gap may cause another problem. In order to achieve a high tuning range (in this case, the ratio of the capacitance with maximum DC bias applied to that with no DC bias), the ratio of the minimum plate separation to the zero-bias plate separation must be large (e.g., 10 times would be desirable). Unfortunately, the minimum gap between the plates (maximum capacitance, and correspondingly, maximum danger of breakdown or xe2x80x9cflash-overxe2x80x9d failure between the plates) is achieved under exactly the wrong bias conditions: when the DC bias voltage is at a maximum.
Some of the deficiencies of the MEMS variable capacitor described above have been addressed in U.S. Pat. No. 6,347,237. In particular, plate separation control has been improved by the addition of an independent mechanical actuator. Plate motion is provided by a mechanical driver, such as a piezoelectric device, which is coupled to one of the capacitor plates. A tuning signal is connected to the mechanical driver to provide control signals for controlling the plate separation. The mechanical driver eliminates the problems associated with microphonics and other external disturbances and thus, control of plate separation is much more precise.
While the mechanically driven MEMS variable capacitor provides extremely high Q values and increased immunity to external disturbances, these improvements come with a price. In particular, the piezoelectric material required for the mechanical driver is relatively large, having a length of approximately 5 mm. This length may be reduced to approximately 3 mm through folding of the piezoelectric material. The overall length, however, is significantly large when compared to other integrated components. Furthermore, the mechanical driver requires precision mechanical fabrication and assembly, thus adding cost and time to the manufacturing process.
Accordingly, there is a need in the art for a tunable filter that is compact in size. Additionally, it would be advantageous to provide such a filter with accurate and repeatable cutoff frequencies and low insertion losses. It would also be advantageous to provide such a filter that is easily manufactured.
In the light of the foregoing, one aspect of the invention relates to an integrated circuit tunable filter, which includes a substrate, an input line on the substrate, an output line on the substrate, a plurality of tuning stubs on the substrate and a plurality of resonators on the substrate. At least one resonator is operatively coupled to the input line and at least one resonator is operatively coupled to the output line, and the plurality of resonators include at least one MEMS switch, wherein the at least one MEMS switch connects and disconnects the resonator to at least one of the plurality of tuning stubs to adjust the center frequency of the tunable filter.
A second aspect of the invention relates to an integrated circuit tunable band-pass filter, which includes a substrate, an input line on the substrate, an output line on the substrate, a plurality of interdigitated stripline resonators on the substrate and a plurality of switch-capacitor groups on the substrate. At least one interdigitated stripline resonator is connected to the input line and at least one interdigitated stripline resonator is connected to the output line. Each switch-capacitor group includes a capacitor connected in series to a micro electro mechanical system (MEMS) switch, and each MEMS switch includes a control signal to connect or disconnect the respective switch-capacitor group from one of the plurality of interdigitated stripline resonators.
A third aspect of the invention relates to an integrated circuit tunable band-stop filter, which includes a substrate, an input line on the substrate, an output line on the substrate, a transmission line on the substrate, a plurality of switch-capacitor groups on the substrate, and a plurality of transmission line resonators on the substrate. The transmission line is operatively coupled to the input line and the output line, and each switch-capacitor group includes a capacitor connected in series to a micro electro mechanical system (MEMS) switch, and each MEMS switch includes a control signal to connect or disconnect the respective switch-capacitor group from the transmission line. Each transmission line resonator is coupled to the transmission line through one of the plurality of switch-capacitor groups.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.